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Why not simply put a set of duallies under the rear and take to an auto engineer to have increased. There is nothing else done between the 2500 and the 3500 maybe slightly heavier rear springs. Your 2500 can tow 4.5 t on the ball and 7.5t 5th wheeler. Don't forget a light truck endorsement will be required for vehicles over 4.5t. From memory the 2500 comes in at 4536kgs unless you have had it derated

To upgrade to 4495kg GVM on a 2500, you need to bring the rear springs to 3500 specifications. This requires the addition of 3 extra leaves per set. The LS11 certification, can than be carried out by a qualified mechanical engineer in Queensland.

I did all the work myself, and had the springs upgraded at a local spring works, than refitted them. (they are heavy)

I already had airbags fitted. When the upgraded springs were fitted I also needed to install a 25mm spacer for the air bags.

The certification for mine was carried out before the latest government saga's but should still be able to be carried out, you also need to ensure your brakes are up to standard as the vehicle is put through a rigorous series of brake tests as part of the upgrade process.

Some information attached, I used for the engineer to confirm my research.

You need the specifications of the 3500 spring pack (attached), when you get the 3 new leaves installed they need to reset the springs to the 3500 specifications.